1. Field of the Invention
The present invention relates to a control system for actuating vehicle safety devices such as an air-bag, a safety belt tightening device and the like in a motor vehicle.
2. Description of the Prior Art
For ensuring the safety of motor vehicle passengers, there have been developed various vehicle safety devices such as the air-bag, the safety belt tightening device and the like. For these devices to work effectively, it is necessary to detect when a collision has occurred as soon as possible, and to actuate the safety device reliably when a collision has occurred. On the other hard, it should be ensured the safety device will not be actuated owing to an erroneous detecting or discriminating operation.
Various control systems for actuating vehicle safety devices have been proposed for meeting these requirements. For example, U.S. Pat. No. 3,911,391 discloses a control device in which the output from an acceleration sensor is integrated when the output level of the acceleration sensor exceeds a predetermined level to obtain information concerning the vehicle speed thereafter and it is determined that the vehicle has collided with an obstruction when the decrease in the vehicle speed reaches a prescribed level. Thus, in the disclosed control device, the operation necessary for ensuring the safety of the operator and passengers of the motor vehicle is actuated when a sudden decrease in the vehicle speed has occurred.
However, in the case of a vehicle with a crushable structure in which the impact incurred at the time of a collision can be absorbed to soften the impact, the impact is absorbed during the initial period of the collision. As a result, the output of an acceleration sensor located in the passenger compartment is likely to be small just after a collision has occurred, and the acceleration sensor produces a large output only later, after sudden acceleration or deceleration of the vehicle body occurs Therefore, in such a case, if it is determined that the vehicle has collided with an obstruction at the time the output level of the acceleration sensor reaches a predetermined level, the safety device may be actuated with a relatively large delay time. As a result, the movement speed of the body of the driver or passengers per unit time has already become large and the quick displacement of the head of the car occupants may have already started. Accordingly, when the safety device is actuated it is very difficult to keep the displacement of the head within a predetermined range which is required for ensuring the safety of the car occupants. To overcome this problem, if the prescribed level described above is lowered, it is liable to cause an erroneous discrimination and the reliability of the control device will be degraded
Furthermore, in the prior art there has been proposed a control system in which a switch type sensor is placed at the forward and central portion of the vehicle in order to detect vehicle collision as soon as possible. However, the reliability of the sensor of this type is not sufficient and the sensor or the wiring therefor will be destroyed when the vehicle collides with an obstruction.